Doubleclutch downshifting

[ClutchFork]

Why is the guy in Fast and Furious looking in the back seat instead of having his eyes on the road? (BTW, I never saw the movie).

[shiftmate]

Now that I understand the doubleclutch downshift, I've been trying to implement it more often and one of the things I'm discovering is that I need to be a little quicker about it because if I do it slowly, after blipping the throttle in neutral and downshifting, the engine speed will drop quickly enough that the crankshaft is spinning slower than the input shaft (and everything else downstream to the wheels) and I either have to slip the clutch or blip the throttle again.

I suppose I could blip the throttle harder in neutral to buy a little more time to engage the clutch in the lower gear before the revs drop below input shaft speed.

[theholycow]

I suppose I could blip the throttle harder in neutral to buy a little more time to engage the clutch in the lower gear before the revs drop below input shaft speed.

Or you could hold it instead of blipping it.

If you do decide to speed stuff up and stick with blipping, remember that the parts that you need to rush are the parts after the blip. That's a pretty simple set of movements; slap clutch pedal to the floor, move shifter one notch (hand should already have been prepared for this), dump clutch and step back on the accelerator. Really, the idea here is to prepare all of those things before you blip; wind yourself up for those things, blip, and proceed.

And yes, you may indeed need to blip it more than you thought if it still takes too long. However, remember that it needs to be perfect for the moment that you shift, not the moment you engage the clutch; when you engage the clutch it's time to get back on the accelerator no matter what. When you double clutch, the blip is for the benefit of the shifter.

[shiftmate]

Or you could hold it instead of blipping it.

Good point.

If you do decide to speed stuff up and stick with blipping, remember that the parts that you need to rush are the parts after the blip. That's a pretty simple set of movements; slap clutch pedal to the floor, move shifter one notch (hand should already have been prepared for this), dump clutch and step back on the accelerator.

If my blip isn't perfect, then dumping the clutch is going to result in the car jerking. But yeah, all the videos of doubleclutch shifting I've seen show the driver dumping the clutch (both times).

I think the second clutch pedal release is part of why I'm slow to complete a doubleclutch - because I don't yet have the precise clutch friction point in muscle memory yet, I have to hunt for it and by the time I find it, the revs have usually dropped too far and the input shaft has to accelerate the crankshaft.

And yes, you may indeed need to blip it more than you thought if it still takes too long. However, remember that it needs to be perfect for the moment that you shift, not the moment you engage the clutch; when you engage the clutch it's time to get back on the accelerator no matter what. When you double clutch, the blip is for the benefit of the shifter.

Doesn't that blip serve two purposes, and not just one?
1. accelerate gears to allow mating of collar to lower gear without waiting for synchronizer to do that job
2. accelerate engine (rev match)

While a single clutch rev match would serve only the second purpose.

[IMBoring25]

There's one issue. If you're blipping, you will typically need two blips...One for the double-clutch and another for the rev-match. The engine speed just falls too quickly in most vehicles for one blip to be sufficient without badly overrevving the double-clutch or badly underrevving the rev-match. You can also use steady throttle but I've never liked that.

[theholycow]

I think the second clutch pedal release is part of why I'm slow to complete a doubleclutch - because I don't yet have the precise clutch friction point in muscle memory yet, I have to hunt for it and by the time I find it, the revs have usually dropped too far and the input shaft has to accelerate the crankshaft.

It'll improve with practice -- when you get better at hitting your target RPM just right. At that point you'll get to dump the clutch rather than having to search for friction, though of course your practice will also help you get friction more quickly and accurately too.

There's one issue. If you're blipping, you will typically need two blips...One for the double-clutch and another for the rev-match. The engine speed just falls too quickly in most vehicles for one blip to be sufficient without badly overrevving the double-clutch or badly underrevving the rev-match. You can also use steady throttle but I've never liked that.

^What he said. If your blip is perfect for the synchronizer, it'll be too low by the time you dump the clutch unless you're really fast (which, with practice, you can be). However, since you already need to get back on the accelerator anyway (you're doing this for the sake of accelerating, right?), all you have to do is get on it slightly earlier and that takes care of both your second blip and your resumption of acceleration.

[shiftmate]

^What he said. If your blip is perfect for the synchronizer, it'll be too low by the time you dump the clutch unless you're really fast (which, with practice, you can be). However, since you already need to get back on the accelerator anyway (you're doing this for the sake of accelerating, right?), all you have to do is get on it slightly earlier and that takes care of both your second blip and your resumption of acceleration.

No, not all my doubleclutch downshifts are for immediate acceleration purposes.

For example, heading into a turn (haven't started learning heel/toe yet), I may want to downshift before the turn so that I'm in 2nd gear before the apex of the turn.

Another example would be a highway, where there is a minor slowdown, say due to rubbernecking or an onramp merge, and I want to downshift to 4th gear just to keep the revs from dropping too low, not so I can immediately accelerate.

[IMBoring25]

Even in those situations I get back on the throttle and then roll off of it in order to get a smooth shift without asking the clutch to do all the work.

[shiftmate]

There's one issue. If you're blipping, you will typically need two blips...One for the double-clutch and another for the rev-match. The engine speed just falls too quickly in most vehicles for one blip to be sufficient without badly overrevving the double-clutch or badly underrevving the rev-match. You can also use steady throttle but I've never liked that.

This makes sense, but from the videos I've seen, it seems the drivers are blipping once.

Here's an example - go to the 40s and 1m07s marks:


Of course, he's also performing this very quickly.

In both cases, he is doubleclutch downshifting in the process of slowing down, so he's not hitting the throttle during/after re-engaging the clutch. Though it does seem he is slipping the clutch - perhaps because revs have slipped too low to dump the clutch (without jerking the car).

[IMBoring25]

With an S2000 he's getting the shifts done pretty fast, plus the camera is rigidly mounted so you can't see how the car is reacting. Try that in my truck and you'd want a HANS device.

[theholycow]

If you're not doing it for acceleration and don't intend to get back on the accelerator at least enough for neutral throttle to avoid engine braking, you're doing it for engine braking. In that case your clutch can do the job of raising the engine speed by a few RPM, if you don't want to do a second blip.

[shiftmate]

If you're not doing it for acceleration and don't intend to get back on the accelerator at least enough for neutral throttle to avoid engine braking, you're doing it for engine braking. In that case your clutch can do the job of raising the engine speed by a few RPM, if you don't want to do a second blip.

True, but you have more clutch wear.

I think I'll work on speeding up the movements after the first throttle blip - i.e., disengaging the clutch immediately after the blip, shifting into the lower gear (though it seems the synchronizers are doing a little work here as I typically have to push on the shifter and wait a fraction of a second for it to easily drop into the next gear; I suppose this is because my throttle blip isn't quite accurate, which is another thing to work on), finding the FP and getting off the clutch pedal.

[shiftmate]

THC, thinking some more about your suggestion to just hold the throttle down, I guess that would work just fine. Since the drive shaft and wheels are decoupled from the input and intermediate shafts, the difference in load on the engine between clutch engaged and clutch disengaged is probably negligible (just the input and intermediate shafts), so holding throttle steady while depressing the clutch pedal and shifting into the lower gear wouldn't stress anything or send the engine revving higher. And then I could really just dump the clutch (and immediately get off the throttle if I didn't want to accelerate or maintain speed).

[Jastreb]

If you're not doing it for acceleration and don't intend to get back on the accelerator at least enough for neutral throttle to avoid engine braking, you're doing it for engine braking. In that case your clutch can do the job of raising the engine speed by a few RPM, if you don't want to do a second blip.

I think it is possible to be perfect with one blip, if you put the shifter in gear at the same time as you start releasing the clutch. There doesn't have to be a long time gap.

I do all my DC downshifts with a single blip. I guess I tend to slightly overrev for the synchro (still helps though) but try to make sure my clutch release is at the right engine rpm.

[theholycow]

I think it is possible to be perfect with one blip, if you put the shifter in gear at the same time as you start releasing the clutch.

The more you overlap, the more it's like not using the clutch pedal at all. That's fine if your rev match is perfect...maybe not so great otherwise.

There doesn't have to be a long time gap.

Indeed, if the time gap is very short then one blip is likely to be close enough. I get the feeling that some folks in here have difficulty coordinating their actions to minimize that gap, for lack of practice or for lack of understanding.

I do a single blip, but then I also get back on it to accelerate anyway. When engine braking I do a single blip and, if my timing were to be imperfect, I'd optimize for the synchro and let the clutch take up the difference.